Herbicide and fertilizer application trials to improve production in Giant rat’s tail grass (GRT) infested pastures.

Abstract

Giant rat’s tail grass (GRT) (Sporobolus natalensis (Steud.) T.Durand & Schinz and S. pyramidalis P.Beauv.) is an invasive weed of pastures. Conventional control efforts for GRT centre on pasture management, the use of chemical and mechanical control and plant competition. To improve management options, recent studies in south-east Queensland have focused on (a) better understanding the residual effects of the most widely used herbicide (flupropanate) and (b) fertilization to determine if it can enhance forage quality and utilization of GRT, particularly in high rainfall environments. In the herbicide trial, granular or liquid flupropanate were applied at label recommendation of 1500 g a.i. ha-1, to mature GRT plants growing in one of five soil types and to pots containing soil only. Residue levels were monitored annually in both soil and in GRT for four years. In an initial ungrazed fertiliser trial eight rates of nitrogen (0 – 300 kg N ha-1) were applied to a GRT infested setaria (Setaria sphacelata (Schumach.) Stapf & C.E.Hubb.) pasture. A second integrated trial was testing the combination of four fertiliser (0, 50, 100 and 200 kg N ha-1) and two herbicide applications (± herbicide) under grazed conditions.

Irrespective of soil type, GRT plants in the herbicide trial contained 22±0.3% (granular) and 31±1.3% (liquid) of the applied flupropanate after 12 months, with levels dropping to <5% after 24 months. Flupropanate in the corresponding soil pots were 20±1.7% (granular) and 7±1.3% (liquid) after 12 months, with similar levels recorded after 24 months. No significant difference was observed between flupropanate formulations when applied to bare soil at 12 (83±3.3%) and 24 (73±1.8%) months after application. Whilst a range of plant response measurements are being undertaken in the fertilizer trials, in this paper we focus on changes in leaf tensile strength and differences in grazing patterns. GRT leaf material was found to have a much higher tensile strength than setaria, and it increased with maturity for GRT but not setaria. Increased fertilisation had a weak negative correlation (P=0.065) with leaf tensile strength. In the grazed trial, irrespective of fertilizer regimes, cattle introduced to 5-week-old regrowth tended to heavily graze both GRT and setaria over the first 2 weeks, particularly setaria which was grazed lower (24.1 cm) than GRT (38 cm). This has allowed wick wiper applications of a flupropanate + glyphosate based mixture to be applied to the taller GRT plants, with efficacy and non-target damage assessments the focus of on-going monitoring.